Born in Boston, Marjorie Harris moved to Florida as a young girl. Her early interest in nature was cultivated by the unspoiled beauty of Lee County in southwest Florida and her parents considerable knowledge of wildlife. Her parents, she said, “knew the answers to the questions I had about the natural world.” In 1936, she received a B.S. in zoology from Florida State College for Women; in 1942, she received an M.A. from the University of Florida. After completing her undergraduate degree, she became a wildlife technician in Florida for the federal government-the first woman to hold such a position. In 1937 she married Archie Carr, who would become a world-renowned biologist and conservationist. They had five children.

During the 1960s, Marjorie Harris grew active in conservation. Her early efforts led to the formation of Payne's Prairie Wildlife Refuge, now a major state park in Florida. She was also involved in various efforts to educate the public about the natural world and instill in them a desire to preserve it.

Her most significant accomplishment was the long battle to stop the construction of the Cross Florida Barge Canal, a mammoth, multi-million dollar project traversing central Florida that threatened the area's ecosystem. Marjorie Carr began fighting the project in 1962, when it was first announced. She eventually founded the Florida Defenders of the Environment in 1969, an organization devoted to defeating the project. In 1970, the organization presented a report called an “environmental impact statement” that provided thorough scientific, technical, and economic research detailing the environmental damage and economic unsoundness of the canal project. It was the first such report submitted by a citizen's group—the group presented hard, convincing scientific and economic evidence to persuade the government to halt the project. It worked, and in 1971 the project was abandoned. Lawton Chiles, Florida's governor at the time, remarked that “few people have Marjorie's intelligence, perseverance, scientifically based understanding of the environment, and ability to lead citizens. Her efforts to deauthorize the Cross Florida Barge Canal began with one person and ended years later with thousands of citizens convincing their elected representatives of the detrimental nature of this project.”

Today the area that was set aside for the canal is a recreational area preserving a variety natural habitats. In honor of Marjorie Carr's conservation efforts, it is now called the Marjorie Harris Carr Cross Florida Greenway.